Telephone-exchange system



Aug. 21, 1928. 1,681,221

P. P. coGGlNs TELEPHONE EXCHANGE SYSTEM Filed Sept. 16. 1927 By Arron/vir il Patented Aug. 21, 1928i.

UNITED STATES PATENT OFFICE- v v BAUL 1r. coefGrrlxTs,vv 0E NEWARK-NEW JERSEY, AssIeNoR To AMERICAN rEiLErrIoiiE,

AND TELEGEAPH oor/frans?, A eoEroEArIoN EY NEW YORK., 1

` `1 ,'rELErnonE-EXCEANGE sYsrEivr.'

= lApplication filed September This invention relates to telephone eX- change syst-ems and, more particularly to trunking arrangements therefor, its obj ect be-' ing to increase the feiiciciencyof .trunkmg' `meanszwithorut impairing the 'serViceQren-f 'dered l Heretofore various methods of slippingand grading multiples for the purpose of equalizing traiiic have beenproposcd and certain of these schemes have 'been"commercially adopted. The most common term otigraded mul- Vtiple is that inwhich a certain number of i groups of .incoming lines are eaelrprovided out-lets. The present invention is a novel with a number of individual first choice outlets `and a number of common second choice variation'ot this scheme and consists essen- Y tial-ly in making the second choice outlets common to a subgroup of each group of incominglinesmot only of the saidcertain number of groups but to a plurality of such certain numbers of groups.`` Bymeans` otthis arrangement, ,the assumptions on :which one oat-the approximate mathematical formules representation of a'standard scheme of graded multiple in commercial use and Fig. 2 is' a schematic representation of agraded mul-V tiple in .-accordancewith vthe present invention;

Numerous attempts have from time to time been made to devise multiple arrangements oic trunks whereby the probabilityof any one group becoming' overloaded will bei-equal to the probability of any other group ybecoming overloaded. A fair discussion of some of these arrangements is contained 1n Aitkensf book on AutomaticL Telephone Systems, Vol. 3, page 299 et seq. andan numerous patents contained in thefprior art. among these patents, No. 1,488,691'to Lundquistshows the graded multiple worked out more elaborately than is necessary but nevertheless `showing fundamentally the standard commercial arrangement. Y f

u Clausen-in` his Patent No. v1.192.165 states thatzwhere trunk hunting switches have ac- 16,1927. serial NO ziasoeL cess to a group ofregularly disposed trunks that the tratlic is `distributed about as follows:

v1st trunk 9th trunk 8th trunk 2 10th trunk Realization of these facts has given rise to schemes known as the graded multiple in y which the number of switches having access to the various trunksincreases with the disi tance' over which the switch has to travel. Probably the most'complete graded multiple tliat'has been .worked out is disclosed in `PatentNo. 1,311,432 to Aitken and also described in hisjbook o-n Automatic Telephone Sys-v, .temsW-abovenoted. AGrraded multiples 'off this nature, however, are opento the'very Vserious Vobjection that from the very random nature of the trunk appearances the tracing oft' the call becomes dilicult and this object-ion isconsidered ,great enough to preclude theY use of such a scheme in most commercial telephone systems. Y f

The ordinary type of graded multiple shown or-instanceinPatent No'. 1,488,691 to Lundquist comprisesessentially a. number number of common vsecond choice trunks. Thisscheme is. to al certain extent, a compromisefor'while it does noty grade the multiple according to lthe most complete theoretical considerations lit actually results in a saving of trunks and still renders` calls capable of .l

being easily traced.

VA, number of inore 'or lessapproximate formulae have been proposed for calculating this type of graded multiple.V Some ofthem areI more complicated than others, and empirical determination of the efficiency of a.' wide range of gradedmultiple arrangements is extremely laborious and has not yet been. carried out at any great'length. One ot the i ioe simpler lapproximate formulae is the result of a combined application of the .well known Erlang B` formula and the Poisson summation.. The Erlang B formula is` based jon the y l i ot individual first choice trunks and a certain to complicate the trafiic. Y The Poisson sum- .oytions of Poissons probability summation assumption that in the group of trunks .undery consideration such calls as fail to find an idle trunk within that trunk immediately will not reappear before the group.` This Vvformula is applied tothe'first choice trunks individual tov a group of incoming lines. This assumption may fairly be made because it is assumed that any call not being absorbed in the first choice individual trunks will be taken care of in the vsecond choice common trunks. When, however, we come to calculate the traffic for the common trunks, that.

is to say the traffic which overflows from the individual trunks, we should takeV into ac` count the fact that a lostrv call may reappear mation formula takes this fact into account.

long run will be lost.

It is based, however, on the assumption that the traffic originating before the group .to which it is applied originates at random in time and from allarge number of independent sources. These formulae are as follows:

Vhere P the proportion of lost calls. For instance, if P results'in the figure .002l it means that two calls outpof a. thousand in the c=the average load per unit of timevexpressed as the number of lcalls. submitted per groupper unit of time,

the unit of time beingV equal to rtheaverage Y 'l holding time.

. ,vided to carry the traffic a and e==2,718,1the f base of Napierian logarithms.v The Erlang m=the number of trunks pro- B formula may be found in The Post-@Hice vElectrical Engineers Journal for January 1918 (pages189 to 197). Erlangs formula is 'also' given in Smith and Campbells Automatic Telephony 2nd edition,page 388 and the bibliography on pages 102 and 403 gives references to other publications where this formula is discussed. -Discussions of the Poisson formula may be found, first, in a paper printed in the Bell'System Technical JournalNovember, 1922, entitled The theory of probabilities applied toftelephonel trunking problems by Edward G. Molina; second, a

' paper printed in the Bell System Technical l. Journal, J anuai'y,1923, entitled .Probability curves showing Poissons exponential summationl by George A. Campbell; and third, a paper printed in the Bell VSystem Technical Journal, October, '1926,fentitled Applicav by Francis Thorndike. i

The present invention is'the result of'a clon-v templation of these formulae and an effort to multiple trunks in such a manner that the as-` sumption on which. they arefbased will. more plurality ofsuch groups.

closely correspond with the equipment arrangement. The main effort is to make the calls submitted to any common trunk group originate more at random from a larger number of independentl sources than heretofore. Essentially the present scheme consists in a novel modification of the standard grading shownin Fig. 1 and comprises the use of several groups of common trunks each group being accessible to a subgroup of the group of incominglines having access to the individual outgoing lines and being, multipled vnot only to tlie groups heretofore served Vbut. to a Calculations based on thecombined appli- 180 'cation of the Erlang B formula and the Pois'- sonformula indicate that the call, carrying capacity ofthe proposed rearranged graded `multiple should be Vtheoretically yconsiderably greater 'than that ofV a correspondingstandard graded arrangement as estimated by theoretical formulae and limited empircal observations. This is because lthe common trunks are made accessible to calls a rather larger proportion of the time Vby means `of their appearing before a number of'relatively independent subgroups of selectives.

Looking now at Figyl it'A will be noted that there arel several vgroups of incoming linesjrepresentedv by verticalv arrows. Indi- 'vidualtothe incoming lines of group 1 is a group of outgoing trunks 3 and individual to the group 2 of incoming lines isa group 4 of outgoing lines. A group -5 of outgoing lines, is common' to the incoming lines o both group 1.and fgroup 2. Grroup 1 and -groupQ constitute what we may hereterm `a master group indicated bythe bracket 6.

lines. For instance, group 27 of the common outgoing trunks is associated with subgroup 9 of group 15, subgroup" 10 of group 16,subgroup 11 of group 17, etc. ln like manner, group Q8 of common outgoing trunks is as-v sociatedvwith subgroup of group 15 of Vthe incoming lines, subgroup 31 of group 16 of the incoming lines, etc., and group 29 of the 'common outgoing -trunks is associated with .subgroup 32 of groupvl of the' incom` infrlines, subgroup 33 of group 16 of the inoming lines, etc. l

Thus, it will be seen that lthe common trunks, although s till the same in number and being still accessible to the samelnumberv of lines, nevertheless receive their tiafie more at random from a large number of iiidependent subgroups Without requiring that aI given selector have access to any more trunks than in the former standard arrangement. Therefore, such an varrangement as the one proposed may be expected to handle i Fig. 2 is such that the effect oii such unbal-V ances Would be minimized. ,For example, if group 15 of incoming lines consistently pro g vided more traflictlian some ot-the otherA groups, it Would, to a certain extent, overload its individual Grou i of outgoinol trunks FJ l t- 21 but would sharethe common trunks with all theV rest of the groups 16 to 20, among which Would be the lighter loaded ones. i On, the other hand, if it should happen that, torf example, the particular subgroups of incoming lines 9, 10, 11, 12, 13 and lshould bey unusually heavily loaded, the first choice individual trunks used by these subgroups would also be shared with less busy subgroups, so that by the time the tra'tiic reached the common trunks it would be considerably smoothed out and. no essential overload Would result on any one of the groups of common trunks. This feature is of value in simplifying the administra-tion of graded trunk groups.

hat is claimed is:

1. The combination of a plurality of groups of incoming lines, a groupof outgoing lines individualto each said group .of incoming lines, `and a group of outgoing lines common to one subgroup only of each of said groups of incoming lines.

2.*The Y combination of a plurality of groups of incoming lines,a group oit outgoing lines individual to each `said group of incoming lines, and a plurality of groups of outgoing lines each group being common to one subgroup only ot each oi' said groups of incoming lines.

3. The combination of a master 0roup of incoming lines divided into groups and subgroups, a group of outgoing lines individual to'` each said group ci incoming lines and'common to all the subgroups thereof and a group oi outgoing lines common to said master group ot incoming lines and individual to one subgroup only of each said group of incoming lines.

fl. The combination of a plurality of groups of incoming lines, a plurality of first choice outlets individual to` each said group and a plurality of second choice outlets common to one subgroup only of each of said` groups. Y

5. The combination of a plurality of groups of incoming lines, a plurality of first choice outlets individual to each saidg-roup v and a different plurality of second choice outlets for each subgroup only of each of Vsaid groups, each said plurality of second choice outlets being common to all said plurality of groups of incoming lines.

In testimony Wiereoi', Il have signed my naine to this specification this 14th day of September, 1927. Y i

' PAUL` r. cocaine. 

